Large scale applications of electronics systems generally require direct current (DC) power provided by a rectifier system which rectifies three phase alternating current (AC) from an AC power source, such as an AC generator.
For a given AC input voltage, such a rectifier system must be able to provide a specific DC voltage output, while minimizing harmonics induced in the AC power source over a wide range of load currents. A rectifier system has numerous shipboard and aircraft applications, therefore such a system should be as small, lightweight, and efficient as possible to minimize space, weight, and ventilation requirements.
Excessive harmonic currents induced by rectifier systems produce numerous deleterious effects, particularly in shipboard applications. These effects may include significant electronic system performance degradation, electromagnetic interference, excessive shipboard hull currents, AC generator rotor overheating, and harmonic-induced torques in AC generators resulting in undesirable vibration modes and system specific "sound signatures."
Harmonic currents are the direct result of the process of rectifying AC power to produce DC power. These harmonic currents can be reduced by increasing the number of AC phases being rectified. Numerous methods exist for converting three phase ac input power into multiphase output power suitable for rectification. Most use multiphase transformers having three phase primary windings coupled to multiple wye or delta secondary windings. Where isolation is not required, autotransformers, as discussed by Michael L. Williams in "Reduction of Line Current Harmonics in 3-Phase Off-Line Rectifier Systems by Use of an Efficient 3-phase to 9-phase Auto-Transformer Conversion Technique," Naval Sea Systems Command, Apr. 12, 1979 have been used. These multiphase methods reduce the harmonic currents generated by the rectifier system, but require resistive and/or inductive tuning of the multiphase transformers to reduce harmonic currents to acceptable levels. This tuning reduces the transformer efficiency resulting in larger, heavier transformers and increased ventilation requirements. Moreover, autotransformers do not allow an output voltage to be specified for a given input voltage.
Accordingly, there exists a need for a multiphase transformer, and a method for designing a multiphase transformer that provides precisely controlled multiphase output resulting in low harmonic currents over a wide range of load currents without requiring resistive and inductive tuning when applied to a rectifier system, and that provides a specified output voltage for a given input voltage.